Category: SCIENCE

Everything acts like a wave while it propagates, but behaves like a particle whenever it interacts. The origins of this duality go way back.

One of the most powerful, yet counterintuitive, ideas in all of physics is wave-particle duality. It states that whenever a quantum propagates through space freely, without being observed-and-measured, it exhibits wave-like behavior, doing things like diffracting and interfering not only with other quanta, but with itself. However, whenever that very same quantum is observed-and-measured, or compelled to interact with another quantum in a fashion that reveals its quantum state, it loses its wave-like characteristics and instead behaves like a particle. First discovered in the early 20th century in experiments involving light, it’s now known to apply to all quanta, including electrons and even composite particles such as atomic nuclei.

But the story of how we discovered wave-particle duality doesn’t begin and end in the early 20th century, but rather goes back hundreds of years: to the time of Isaac Newton. It all began with an argument over the nature of light, one that went unresolved (despite both sides declaring “victory” at various times) until we came to understand the bizarre quantum nature of reality. While wave-particle duality owes its origin to the quantum nature of the Universe, the human story of how we revealed it was full of important steps and missteps, driven at all times by the only source of information that matters: experiments and direct observations. Here’s how we finally arrived at our modern picture of reality.

Huygens: light is a wave

Picture a wave propagating through water, such as in the ocean: it appears to move linearly, at a particular speed and with a particular height, only to change and crash against the shore as the water’s depth lessens. Back in 1678, Dutch scientist Christiaan Huygens recognized that these waves could be treated — rather than as linear, coherent entities — as a sum of an infinite number of spherical waves, where each…

Almost everything we can observe and measure follows what’s known as a normal distribution, or a Bell curve. There’s a profound reason why.

Whenever a baby is born, doctors measure a number of vital statistics about them: height, weight, number of fingers-and-toes, etc. A newborn child is generally considered healthy if they fall somewhere near the average in all of those categories, with a normal, healthy height and weight, and with 10 fingers-and-toes apiece. Sometimes, a child will have an unusually low or high height or weight, or greater or fewer than 10 fingers-and-toes, and the doctors will want to monitor them, ensuring that “not normal” doesn’t imply a problem. However, it turns out that there being an idea of “normal,” where “normal” means the most common set of outcomes, is universal to practically anything we dare to measure in large quantities.

It’s easy to imagine for something like height, as while there are many full grown adults of average height, there are fewer numbers of tall people and short people, and even fewer numbers of extremely tall or extremely short people. But in nature, practically anything that you measure will wind up following a Bell curve distribution, also known as a normal or Gaussian distribution. Why is that? That’s what L Viswanathan wants to know, writing in to ask:

“[I] read your recent post about [the] Fibonacci series, which prompted this question. We know that most of the phenomena in nature follows the normal distribution curve. But why? Could you please explain?”

It’s a relatively simple question, but the answer is one of great mathematical profundity. Here’s the story behind it.

The starting point, in anything that’s going to follow some sort of distribution, is what’s known as a random variable. It could be:

• whether a coin lands heads or tails,
• whether a rolled die lands on a 1, 2, 3, 4, 5, or 6,
• what your measurement error is when you measure…

In the early stages of our Solar System, there were three life-friendly planets: Venus, Earth, and Mars. Only Earth thrived. Here’s why.

If you could travel back in time to the early stages of the Solar System, some 4.5 billion years ago, you wouldn’t simply find the one life-friendly world you’d expect in the form of planet Earth. Instead, there would have been three worlds with similarly life-friendly conditions: Venus, Earth, and Mars. In terms of the physical conditions they possessed, all three of them looked very similar from a planetary perspective, as they all had:

• substantial surface gravity,
• copious amounts of volcanic activity,
• and atmospheres similar to Earth’s in thickness and pressure.

They all possessed volcanoes, watery oceans, and complex interactions between the surface, oceans, plus clouds and hazes, enabling these worlds to retain significant amounts of the heat they absorbed from the Sun.

Moreover, at these very early stages, even the compositions of their atmosphere were similar, as they were all rich in molecules like hydrogen, ammonia, methane, nitrogen, and water vapor. For a time, conditions were favorable for life to potentially arise on all three worlds, and indeed it may have arisen on all three at some point in the distant past. However, on all but one of these worlds, it didn’t last. Venus experienced a runaway greenhouse effect, boiling its oceans away and rendering it inferno-like after only a few hundred million years. Mars lasted far longer before becoming inhospitable: perhaps as much as 1.5 billion years. These are the stories of our how planetary neighbors met their respective demises.

It’s remarkable that worlds that, today, are so different from one another might have had such similar histories in their early stages. It wasn’t just Earth, but also Mars, that likely experienced a catastrophic early collisions, with Earth’s creating our Moon and Mars’s creating three moons, the largest of which…

Beyond the planets, stars, and Milky Way lie ultra-distant objects: galaxies and quasars. Here’s how far back we’ve seen throughout history.

The night sky showcases untold astronomical riches.

Closest is our Moon, whose distance was approximated 2000+ years ago.

The Moon and planets sometimes occult stars, demonstrating that stars are farther.

First recorded in 964 CE, the Andromeda galaxy outdistances any object in our Milky Way.

It wasn’t until 1923, however, that measurements of internal variable stars proved its extragalactic nature.

By that time, many more distant objects had been observed.

The Triangulum galaxy, recorded in 1654, is our farthest naked-eye object.

In 1779, spiral galaxy Messier 58 broke that record.

It’s not about particle-antiparticle pairs falling into or escaping from a black hole. A deeper explanation alters our view of reality.

For many good reasons, black holes are among the most studied objects in the entire Universe. Initially predicted back in the late 18th century in the context of Newtonian gravity, black holes were shown to arise in the context of General Relativity as early as 1916. Astrophysically, they can be formed when gas clouds collapse, when the cores of stars implode, or when two neutron stars collide, among other mechanisms. They have been observed via numerous methods: from electromagnetic emissions that arise from matter around them, from the motion of stars or binary companions around them, and from the gravitational waves they emit when two of them merge together.

But perhaps, most remarkably of all, it was shown in the early 1970s that black holes cannot endure forever, but will eventually evaporate due to the continuous spontaneous emission of radiation that emerges from them: Hawking radiation. But how does Hawking radiation truly work? Not as Hawking asserted, unfortunately, which brings us to the question submitted by Leif Koesling:

“Earlier today I was reading your article on Hawking Radiation and was very confused with your explanation.
You mentioned that this particle and antiparticle cancellation was not in fact true, and that it was actually the curvature gradient of the black hole that was creating this Radiation.
Could you maybe explain this? How can the bending of spacetime result in Radiation?”

There’s a lot to consider, but let’s try and make it as clear as possible by beginning from an unusual starting point: empty space.

Most of us, initially, think of empty space as some sort of perfect “emptiness” without anything inside:

• no particles,
• no antiparticles,
• no radiation,
• no fields,
• no curvature of space,

The Universe didn’t begin with a bang, but with an inflationary “whoosh” that came before. Here are the biggest questions that still remain.

Imagine what it must have been like, as it was for so long throughout human history and prehistory, to look up at the wonders of the night sky in ignorance: not knowing what you were seeing or where any of it came from. All you could behold with your eyes were those glittering points of light in the sky: the Moon, the planets, the stars, a few deep-sky objects (or nebulae), and the tapestry of the Milky Way, with no way of knowing what they were made of, where they came from, or what any of it meant.

Today, the story is very different. Nearly all of the night sky objects we can see with our naked eye are objects present within the Milky Way galaxy. A few of those deep-sky objects turn out to be galaxies, with trillions of more galaxies — including small, faint, and ultra-distant ones — observable with superior tools. These galaxies all expand away from one another, with more distant objects expanding at greater speeds than nearer ones.

The expanding Universe swiftly led to the idea of the Big Bang, which was then confirmed and validated. The Big Bang was then modified to include an even earlier stage known as cosmic inflation, which preceded and set up the Big Bang’s initial conditions. That’s the current status of our understanding of the beginning as of today, in early 2024. Here are the biggest questions, both answered and unanswered, that we still have about the earliest phases of our Universe.

The inflationary hot Big Bang

Most of us have heard of the Big Bang: the notion that the Universe began from a very hot, very dense, and very uniform state, and then expanded, cooled, and gravitated, eventually giving rise to:

• protons and neutrons,
• atomic nuclei,
• neutral atoms,
• stars,
• galaxies,

That’s what we’re exploring on this episode of the Starts With a Bang podcast, featuring Ph.D. candidate Dakotah Tyler as our guest this month. By looking at how a hot (but low-mass) Jupiter-sized planet is being photoevaporated by its parent star, we can learn so much about not only the classes of objects we see out there, but even the ones we don’t!

Starts With A Bang is written by Ethan Siegel, Ph.D., author of (affiliate links following) Beyond The Galaxy, Treknology, and The Littlest Girl Goes Inside An Atom. New books, including the Encyclopaedia Cosmologica, are forthcoming!

Early on, only matter and radiation were important for the expanding Universe. After a few billion years, dark energy changed everything.

Imagine looking out at the Universe: beyond the stars of the Milky Way and the nearest galaxies to us, all the way to the most distant objects we can find. When we do exactly that, examining the galaxies, quasars, and other forms of matter that appear billions of light-years away, we’re seeing those objects not as they are today, but as they were in the distant past: back when their light was first emitted. At those earlier times, the Universe was hotter, denser, and filled with smaller, younger, less-evolved galaxies. The light we see from way back in our Universe’s history only arrives at our eyes after journeying across these vast cosmic distances, and only after that light has been stretched by the expanding fabric of space.

It’s precisely these early signals, and the process of how that light gets stretched to longer wavelengths — i.e., redshifted — more severely as we look to more and more distant objects, teach us how the Universe has expanded throughout its history. We learned, by collecting that data, that the Universe wasn’t just expanding, but that distant objects appear to speed up, faster and faster, as they mutually recede from one another: the discovery of the accelerated expansion of the Universe. That’s how we discovered dark energy and measured its properties, changing our conception of the Universe forever. Here’s what it was like when dark energy first took over the expanding Universe.

Imagine that you weren’t a human being, but rather an omniscient being that was not only around during the earliest moments of the hot Big Bang, but were capable of keeping track of two different locations at all times. One of those locations would correspond to the eventual location that our Milky Way would wind up in, today, while the other would correspond to a distant, disconnected galaxy that wasn’t gravitationally bound to anything in the Milky Way’s vicinity: not the Local Group, not…

Valles Marineris is the Solar System’s grandest canyon, many times longer, wider, and deeper than the Grand Canyon. What scarred Mars so?

Here on Earth, one of the greatest geological wonders of all is the Grand Canyon. Carved by the Colorado River over millions of years, which connected multiple older segments of the canyon together, the full extent of this giant, steep-sided valley is now remarkable and impressive. Spanning 446 kilometers (277 miles) in length, the canyon is up to 29 kilometers (18 miles) wide and up to 1.857 kilometers (1.153 miles) deep. The advance, retreat, and melting of glaciers, combined with the release of enormous amounts of water, have exposed a wide variety of rocks formed throughout Earth’s geological history, including formations as many as 2 billion years old.

And yet, the full extent of Earth’s Grand Canyon pales in comparison to the grandest canyon in all the Solar System: Valles Marineris on Mars. Mars, a much smaller planet than Earth with a very different geological past, might not seem like the ideal candidate for such a gigantic feature, and yet not only is it present, it was likely created in a very different fashion than the Grand Canyon was on Earth. But how, precisely, did it form? That’s what Rosa Been wants to know, asking:

“You know how Mars has a huge scar in it? Like 6 miles deep. I was curious what caused it. I’ve heard it could be an asteroid or solar flare kinda thing etc.”

It could have been a lot of things, and in reality, it was probably formed by many processes combined. But the greatest lesson of all for its formation may not come from Earth’s Grand Canyon at all, but a very different feature. Here’s the most likely story we’ve been able to piece together.

What you see, above, is a topographic map of Mars. Although there are many notable features, there are a few prominent ones that are relevant when it comes to discussing the grandest canyon of them all, Valles Marineris, which appears just south of the Martian equator and just…

Our own galaxy, the Milky Way, is both completely normal and absolutely remarkable in a number of ways. Here’s the story of our cosmic home.

The Milky Way galaxy may be just one among trillions present within the observable Universe, but it’s uniquely special for personal reasons to us: it’s our cosmic home. It’s the fertile soil from which our Sun and Solar System, including the bodies that would eventually become planet Earth, sprung some 4.6 billion years ago. All told, it’s composed of a few hundred billion stars, about a trillion solar masses worth of dark matter, a supermassive central black hole of about 4 million solar masses, and a plethora of gas and dust. And that’s no outlier; we’re actually somewhat typical of modern galaxies, with perhaps a hundred billion others similar to our own. We’re neither among the biggest nor the smallest of galaxies, nor are we in an ultra-massive cluster or found in isolation, but rather a modest galaxy group, where we’re the second-largest member.

What does make us special, though, is how evolved our galactic home has become. Some galaxies grow up quickly, exhausting their gaseous fuel and becoming “red and dead” when they lose the ability to form new stars. Some galaxies undergo major mergers, often transforming from gas-rich spirals into gas-free ellipticals in the aftermath of those collisions. Still others experience enormous tidal disruptions, leading to sweeping, distended spiral arms. Not the Milky Way, though. We grew up in exactly a typical fashion. Here’s how we got here.

At the present time, galaxies like the Milky Way are incredibly common. Here are some properties that Milky Way-like galaxies typically display:

• they contain hundreds of billions of stars,
• concentrated into a disk-like, or pancake-like shape,
• surrounded by globular clusters in a halo-like distribution,
• containing spiral arms that extend radially outward for tens of thousands of light-years in either a flocculent or…